ABSTRACT Older adults walk slower and with higher metabolic energy cost than younger adults, changes that reduce independence and quality of life. These functional impairments stem from precipitous reductions in ankle push- off power output that cannot be improved by conventional strength training. Growing evidence reveals that muscle activation patterns tuned to underlying triceps surae (TS) muscle-tendon structural properties facilitate an effective burst of ankle power output during push-off. This study addresses two key questions: (1) Do age- related changes in series-elastic Achilles tendon (AT) structural properties (i.e., stiffness, kT) disrupt the tuned neuromechanical function of the TS with cascading metabolic penalties? and (2) Can donning elastic exoskeletons in parallel with biological TS muscle-tendons alter structural stiffness and improve the neuromechanics and energy cost of walking in older adults? Specific Aim 1 will quantify how aging effects activation-dependent tuning of triceps surae muscle-Achilles tendon interaction dynamics. Using controlled loads on a dynomometer and physiological loads during treadmill walking, we will couple advanced, dual-probe cine ultrasound imaging of TS muscle fascicles and localized AT tissue with novel electromyographic biofeedback to assess individual contributions of muscle versus tendon (kM and kT) to overall TS muscle-tendon stiffness (kMT) over a full landscape of muscle activation. Combined with metabolic measurements, we will test the hypotheses that (1A) older adults have a more complaint AT (i.e., lower kT) than young adults and thus, (1Bi) in isolated muscle contractions at prescribed TS muscle activations, older adults operate at shorter TS muscle fascicle lengths, and (1Bii) during walking at matched speeds, in an attempt to maintain overall kMT, older adults increase TS muscle stiffness (i.e., higher kM) by shifting to higher activations with shorter fascicle lengths than young adults -- with energetic implications at the (1) individual TS muscle (force per unit activation) and (b) whole- body (walking economy) levels. Specific Aim 2 will determine how elastic ankle exoskeletons alter the neuromechanics and energetics of walking in older adults ? from whole-body to individual muscles. Using a novel ankle exoskeleton emulator we will apply a range of exo-tendons (kEXO) in parallel with the TS muscle- tendon (kMT) while older adults walk at a fixed treadmill speed. We will test the hypotheses that (2A) older adults using elastic ankle exoskeletons will demonstrate reduced TS muscle activation and longer TS muscle fascicle operating lengths, and (2B) for older adults, the kEXO that most closely normalizes TS muscle-tendon stiffness (kMT) to that of their size-matched, young counterparts will yield the most youthful walking performance, evidenced by: (i) largest increase in ankle push-off power output and (ii) largest reduction in metabolic energy cost. Ultimately, this work will establish a framework for using ultrasound imaging to guide optimal prescription of assistive devices that can effectively modify the structure of the ankle triceps surae muscle-tendons to improve locomotor function in aging ? an outcome that will have significant positive impact on quality of life for millions.